Carbon nanotube technology is quickly becoming a technological area that is making an impact on the field of microelectronic devices. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Single-wall carbon nanotubes are quasi one-dimensional nanowires, which exhibit either metallic or semiconducting properties, depending upon their chirality and radius. Single-wall nanotubes have been demonstrated as both semiconducting layers in thin film transistors as well as metallic interconnects between metal layers.
One technology uses carbon nanotubes as an electromechanical switch for non-volatile memory devices, where the nanotubes are spin-deposited over a patterned substrate surface. The nanotubes 12 lay over trenches 14 between a first electrode 16 and a second electrode 18 of an integrated circuit 10, as depicted in FIG. 1. The device 20 is switched on by applying a bias to the second electrode 18, and switched off by removing the bias to the second electrode 18, and applying a bias to the first electrode 16.
A two-terminal switching device 20 can be made by over-lapping a metal layer over a nanotube layer 12, as depicted in FIG. 2, where the metal layer is segmented into a first electrode 16 and a second electrode 18.
Current integration schemes for the two-terminal cell 20 consist of patterning the nanotube layer 12 followed by alignment of the second or programming electrode 18 to overlap the nanotube layer 12 ends by a discrete distance, such as from about twenty-five nanometers to about seventy-five nanometers. The accuracy of the alignment is generally limited by the tolerances of the photolithography tool used, suggesting that electron beam or very short wavelength scanners are preferred to achieve the desired overlap.
What is needed, therefore, are alternate methods for the fabrication of nanotube structures that reduce the dependence of the process on critical alignment tools.